Amino acids are the building blocks of proteins. They usually exist as “dipolar

Question

Amino acids are the building blocks of proteins. They usually exist as “dipolar ions” in aqueous solutions. The structure of the dipolar ion of the amino acid alanine is shown below.

Calculate the formal charges on all atoms, except hydrogens, of alanine (6 points). Based on the formal charge calculations, indicate which atoms in the dipolar ion carry formal charges.

What is the overall charge of alanine? (1point)

Explain whether or not you expect the two oxygens to be different. Substantiate your answer suitably (3 points)

Explanation / Answer

Formal charge = Valence Electron in free atom - Total non bonding electron - (Total bonding electron) / 2

Starting from left the formal charge on different atoms are

C-1 atom -> FC = 4 - 0 - 8/2 = 0

C-2 atom -> FC = 4 - 0 - 8/2 = 0

C-3 atom -> FC = 4 - 0 - 8/2 = 0

O-1 atom ( having double bond) -> FC = 6 - 4 - 4/2 = 0

O-2 atom -> FC = 6 - 4 - 2/2 = - 1

N atom -> FC = 5 - 2 - 4/2 = + 1

So O-2 atom and N atom carry -1 and + 1 Formal charges respectively

Overall charge of alanine = -1 + 1 = 0

Yes the two oxygens have different formal charges so the two oxygens will behave differently. O-1 atom has zero Formal charge so it cannot react with H+ ion whereas O-2 atom has -1 Formal charge so it can react with H+ to form OH as the end group.

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